The invention relates to a method for controlling a drivetrain of a vehicle, and especially a utility vehicle provided with a dual clutch transmission. The drivetrain comprises a combustion engine connected and a dual clutch transmission, wherein said dual clutch transmission is provided with a normally closed input clutch, a normally open input clutch and a countershaft. The normally closed input clutch connects the combustion engine with a first input shaft and the normally open input clutch connects the combustion engine with a second input shaft. The countershaft is provided with at least one countershaft sleeve, which countershaft sleeve can be arranged in an engaged and a disengage position. In the engaged position the countershaft sleeve enables a torque transfer from the countershaft to the second input shaft and the countershaft sleeve is in a disengage position when said drivetrain is turned off.
The invention can be applied in heavy-duty vehicles, such as trucks, buses and construction equipment. Although the invention will be described, with respect to a truck, the invention is not restricted to this particular vehicle, but may also be used. In other vehicles such as buses and construction equipment and/or personnel cars.
Dual clutch transmissions have been more and more common for personnel cars and commercial vehicles the last years. A dual clutch transmission is commonly provided with a normally closed (engaged) and a normally open (disengaged) clutch. A spring mechanism is acting upon the normally closed clutch such that it engages if there is no active force holding the clutch open. For the normally open clutch, a spring mechanism acts upon the normally open clutch, such that it disengages if there is no active force holding the clutch closed. The clutches are normally operated by a pressure medium driven actuator. Pressure is build up in the system by a compressor; however during a standstill of the vehicle it can be that the pressure in the system is too low to control the clutches. The normally closed clutch is connected to a first input shaft and the normally open clutch is connected to a second input shaft.
During start up of a combustion engine in a drivetrain, it is preferred that as little inertia as possible is coupled to the crankshaft when cranking the combustion engine. Normally, this is achieved in that the normally closed clutch is opened. However, at occasions when the pressure of the pressure medium is for low to control the normally closed clutch to an open position, the combustion engine has to crank with the normally closed clutch in an engaged position. Whereby, the gearwheels of the dual clutch transmission connected to the input shaft of the normally closed clutch will be adding mass that has to be turned dining cranking of the combustion engine. To minimise the inertia in these situations a connecting sleeve provided in the transmission is disengaged. The connecting sleeve can be arranged in an engaged and a disengage position. In the engaged position the connection sleeve enables an operative connection between said first input shaft to a gearwheel and in said disengaged position gearwheel is disconnected from said first input shaft. To minimise the inertia when cranking the combustion engine with an engaged normally closed clutch, the connection sleeve is positioned in a disengaged position when said combustion engine is turned off. The connection sleeve must be engaged before an update sequence of the clutches slipping points can be performed. When engaging the countershaft sleeve a tooth to tooth situation must be avoided, i.e. avoid that the tooth to tooth situation blocking the engagement of the sleeve. To do this a central synchroniser can be used in order to synchronise the speed of the countershaft sleeve and the gearwheel it connects to. However, using a central synchroniser is complicated, loud and time consuming, which means a delay before a driver can launch the vehicle.
It is desirable to provide a method to control a dual clutch transmission during a start up of a drivetrain, wherein said method facilitates an easy and fast synchronisation and engagement of a connection sleeve.
A drivetrain of which the method according to an aspect of the invention is applied comprises a combustion engine in drive connection with a dual clutch transmission, wherein the dual clutch transmission is provided with a normally closed input clutch and a normally open input clutch. The normally closed input clutch connecting said combustion engine with a first input shaft and the normally open input clutch connecting said combustion engine with a second input shall. The first input shaft is in operative connection with a connection sleeve, wherein the connection sleeve can be arranged in an engaged and a disengaged position. An operative connection is a connection between torque transfer members in a transmission, such as clutches, gearwheels, shafts and sleeves. When a torque transfer member is in operative connection with another torque transfer member, torque can be transferred between the two members. The torque can be transferred directly from one of the members to another or via an number of other torque transfer members in the transmission.
When the connection sleeve is positioned in the engaged position, the connection sleeve enables an operative connection between the first input shaft to a gearwheel and in said disengaged position gearwheel is disconnected from said first input shaft, i.e. there can be no torque transfer between the first input shaft and the gearwheel.
A gearwheel is normally one member in a gear step, gearwheel pair, gear set or gear stage, wherein the at least other member is normally another gearwheel enabling a torque transfer from one shaft to another.
When the drivetrain is turned off the connection sleeve is positioned in its disengaged position. Further, the transmission is shifted into neutral, such that no torque output is available at the output shaft of the transmission, independently of the torque at the input shafts. The method is automatically initiated at start of said drivetrain, and comprising the steps of;
starting said combustion engine,
controlling said normally open and said normally closed input clutch to an engaged or semi engaged state such that both said first and said second input shaft reaches a predetermined synchronised speed, and thereafter
opening said normally closed input clutch, and
when said connection sleeve reaches an essentially synchronous speed with said gearwheel;
engaging said connection sleeve.
Because the transmission always is controlled such that the connection sleeve is brought to a disengaged position before the drive line is turned off a minimum of inertia is secured even though a control pressure of the normally closed input clutch is below its opening threshold value. Hence, the combustion engine can be cranked with as low inertia contribution from the transmission as possible, even though the normally closed clutch cannot be opened. Further, by always controlling the input clutches such that they are synchronised and thereafter open said normally closed input clutch an essential synchronised speed of the gearwheel and the connection sleeve can be achieved, whereby a locking tooth to tooth situation between the connection sleeve and the gearwheel can be avoided and the sleeve can be engaged with the gearwheel. The engagement of the connection sleeve and the gearwheel is performed before a full synchronisation between the two is reached. By having a slight speed difference between the two a tooth to tooth situation, which prevents an engagement is avoided.
Hence, by performing the method according to the above it is secured the connection sleeve always can be connected to the gearwheel and any tooth to tooth situations are avoided.
In one aspect of the method, before the method step of starting said combustion engine the method comprise the method steps of
checking a control pressure of a clutch actuation mechanism, and
if said control pressure is above a first threshold value;
attempting, to engage said connection sleeve.
By first checking the control pressure of the clutch actuation mechanism it can be determined if an actuation of the input clutches can be performed or not. If the control pressure is above a first threshold value the normally closed input clutch can be controlled to be disengaged. With the normally closed input clutch in a disengaged position, the combustion engine can be cranked with a minimal inertia contribution from the transmission, independently if the connection sleeve is in its engaged or disengaged position, whereby an attempt to engage the connection sleeve is made immediately after the normally closed input clutch has been disengaged. If the connection sleeve can be connected directly, without getting stuck a tooth to tooth situation, the method can be terminated.
In one aspect of the method, between the attempting to engage of the connection sleeve and the starting, of the combustion engine, the method further comprise the steps of;
if said connection sleeve does not engage;
bringing said connection sleeve to its disengaged position; and
if said connection sleeve does engage;
disengaging said normally open input clutch,
starting said combustion engine, and
terminating the method.
By adding these method steps it is secured that the connection sleeve can not engage before a synchronisation process between the connection sleeve and the gearwheel is started. The method becomes as efficient as possible and still as low as possible inertia contribution from the transmission is secured when the combustion engine is cranked. Further, the method secures that a tooth to tooth situation in the connection between the connection sleeve and the gearwheel is solved in a fast and efficient manner.
The invention further concerns a computer program comprising program code means for performing the steps of any aspects of the method, when said program is run on a computer.
The invention further concerns a computer readable medium carrying a computer program comprising program code means for performing the steps of any aspects of the method when said program product is run on a computer.
The invention further concerns control unit for controlling a utility vehicle, the control unit being configured to perform the steps of the method according to any of aspects of the method.